Discharge lamps, such as high-pressure mercury lamps or metal halide lamps, have been used as light sources of a projector. In these discharge lamps, the shape of an electrode changes due to a drop in fusibility resulting from consumption of the electrode by discharge or progress of crystallization of the electrode according to an increase in cumulative lighting time. In addition, when a plurality of projections grows in an electrode tip portion or irregular consumption of the electrode body progresses by those described above, the arc origin moves or the arc length changes. Such phenomena are not desirable because they reduce the brightness of a discharge lamp so that the lifespan of the discharge lamp is reduced.
In order to solve the problem, a discharge lamp lighting device that supplies a driving current, in which a low frequency is intermittently inserted in a steady-state frequency, to a discharge lamp is known (see, e.g., JP-A-2006-332015). In addition, a discharge lamp lighting device that supplies a driving current, in which a direct current is intermittently inserted in a high-frequency alternating current, to a discharge lamp is known (see, e.g., JP-A-1-112698).
However, even if a discharge lamp is driven under the same driving conditions, the growth degree or the melting degree of a projection of an electrode tip portion which becomes an arc origin changes with a deterioration state of the electrode. For example, when the melting degree of an electrode tip portion is not sufficient, a projection of the electrode tip portion may deform. Moreover, when the melting degree of the electrode tip portion is excessive, an electrode material may evaporate excessively to cause blackening or needle-like crystals. Therefore, particular consideration is required in order to maintain the shape of the projection of the electrode tip portion.